The present invention relates to a method for encoding and transmitting video signals, particularly but not exclusively digital, and processing the signals to reduce redundancy, and comprising the use of motion vectors indicating global movements of image parts.
Digital encoding systems for video data are known in which algorithms for reducing the redundancy re used to reduce the amount of transmitted binary data with respect to the amount of source data. An example of a method of this type, called hybrid DCT (Discrete Cosine Transformation), can be found in "Coding Strategies Based on DCT for the Transmission of HDTV", by M. Barbero, S. Cucchi, M Stroppiana, 2nd International Workshop on HDTV, Feb. 29-Mar. 2, 1988, L'Aquila, Italy. The described method can be improved, within the known state of the art, by using the evaluation and then the compensation of motion. In this way the time redundancy is eliminated not just in the motionless parts but also in the parts with determined motion, i.e. non-chaotic motion.
Other similar image coding methods are known, in which a time sub-sampling of the image is provided, where motion compensation can be used to improve the received and reconstructed image quality.
In the above mentioned known methods, the image is divided into sub-parts, such as blocks of 8.times.8 pixels, and for each of those blocks an overall motion vector is determined, i.e. the displacement of that image block, from the preceding frame to the successive one, is estimated in pixel number or a fraction thereof.
The image processing provided in the transmitter uses the information related to the displacement of each image block and transmits the processed image and the motion vector coordinates (vertical component and horizontal component) of each block to the receiver.
The image is reconstructed on the basis of those motion vector coordinates together with further service information supplied by the transmitter. The derivation of the motion vectors and their transmission are performed for each frame.
For a simpler description, the word "frame" will be always mentioned to designate the whole of the elements constituting a single image and both the so called "frame" and the so called "field", particularly in television.
Usually, the motion vectors are evaluated within a window, for example +-15.5 pixel and +-7.5 lines, because it is believed that most of the motion in an image is less than 16 pixels and 8 lines per frame.
The coordinates of those vectors can be transmitted, in the simplest way, for each image block. For a fixed-length coding, in the case of the above mentioned window, and for a half pixel precision, this would require eleven (6+5=11) bits per image block, i.e. a significant increase of information to be transmitted, such as to limit the advantages of the redundancy reduction obtained with the DCT plus variable-length encoding. Even considering that some values are more probable than others and that the variable-length motion vectors are themselves encoded, no considerable advantages are obtained.
In order to reduce the binary information in the motion vectors transmission, it has been proposed to use a menu-based transmission. For each frame, a fixed number, for example 32, of most frequent motion vectors is selected and transmitted and for each block, the selected motion vector address is set in the menu. In this manner, just 5 bits per block are sufficient, saving 6 bits with respect to the preceding case, but the selection and assignment of the 32 vectors turns out to be quite complex and burdensome.
Still within the aim of reducing the information flow, or the transmitted bit rate, differential motion vector coding has also been proposed, which consists in transmitting only the motion change for each block, with respect to the same block in the preceding frame. Since usually, the motion of an object is sufficiently uniform, those changes are usually small, and the probability density of the transmitted values is quite peaked around zero; hence the variable-length code transmission turns out to be highly efficient, and the saving in transmitted bits is enormous. Unfortunately, as will be evident to the man skilled in the art, this method accumulates error from one frame to the next, and is therefore highly sensitive to even sporadic channel errors.